The present invention relates to patch antennas and in particular relates to a feed for a patch antenna.
Patch antennas comprise one or more conductive rectilinear or ellipsoidal patches supported relative to a ground plane and radiate in a direction substantially perpendicular to the ground plane. Conveniently patch antennas are formed employing microstrip techniques; a dielectric can have a patch printed upon it in a similar fashion to the printing of feed probes employed in layered antennas.
The feed network will, in general, have certain characteristics which must be carefully monitored in order to minimise any adverse effects on the antenna performance. Printed or lumped elements, such as tapered lines or junctions will introduce electrical and physical discontinuities into a feed line. Attenuation due to conductor loss and dielectric loss will reduce the efficiency, and hence the gain, of an antenna. In practice it is rarely possible to eliminate the electrical effects completely by normal matching techniques, resulting in reflection losses, surface-wave loss and spurious radiation. The latter will, in general, be uncontrolled, and is likely to increase co-polar sidelobe levels in some directions, and to increase the total energy in the cross-polar radiation pattern, thereby reducing the antenna gain.
Direct radiation losses and surface-wave losses are eliminated in enclosed triplate and suspended stripline feeds, but any discontinuity causing asymmetry in the cross-section, such as a probe feed to a patch, will introduce losses due to the transfer of energy to a parallel-plate mode propagating between the ground planes. This energy is free to couple to adjacent probes, and may thus ultimately results in spurious radiation. The mode can be strongly attenuated by the use of mode-suppressing pins close to the discontinuity, or by means of microwave-absorbent film or sheet material, but this increases the complexity of the construction.
Coupling to a microstrip patch may be achieved by a variety of means: direct coupling of a microstrip line, gap-coupling and proximity coupling to a microstrip line and probe coupling, for example.
In the case of direct feed line coupling, the feed line is directly coupled to the patch, critical coupling at the resonant frequency may be achieved by one of the three configurations shown in FIGS. 1, 2 and 3. FIG. 1 shows a feed line 2 and a rectangular patch 4, the patch being fed via a quarter-wave transformer (matching section) 6 having a particular impedance from the feed line. FIG. 2 shows an inset feed arrangement 8 which shifts the feed point of a feed line 10 to a lower impedance region inside the patch 12. For some applications, such as dual polarised applications, this cannot be used because of interference caused by the inset area on the patch, because of a cross polar requirement and the patch edges need to be protected. Equivalent circuits are show for these feed arrangements. The feed line 14 can enter at a point about one third of the way along a non-radiating edge of a patch 16, as shown in FIG. 3. Shorter feed lines with lower loss may be possible using this configuration in a corporate feed network, though an aspect ratio of about 1.5 is required to minimise cross polar radiation. Furthermore the microstrip feedline is exposed and also contributes to spurious radiative effects. A dual polarisation capability will also be difficult to achieve for the patches shown in FIGS. 2 and 3, whilst track losses and layout size are problems for the antenna shown in FIG. 1.
Gap and proximity coupling schemes both utilise a narrow gap between a feed line and a resonant patch, FIGS. 4 and 5 show gap 18 and proximity 20 coupling feeds. The width of the gap dictates the strength of the coupling at the resonant frequency. When the feed line and the resonant patch are critically coupled, the latter constitutes a matched termination. Proximity Coupling is a method used for coupling a single feed line to a linear array of resonant patches and is similar to gap feed coupling. In an array configuration, the individual patches do not necessarily need to be matched to the feed line, neither do they have to operate at maximum efficiency. Coupling gaps can be varied to control the proportion of power coupled into the patches, and the patches themselves can have characteristic impedances rather higher than those normally associated with more conventional low-impedance patches.
Probe coupling has been widely employed, particularly for circular patches, an example of which is shown in FIG. 6. The feed 22 lies behind the radiating patch 24 which is supported on a dielectric substrate 26 which has a ground plane 28 on its anterior surface and therefore does not itself contribute any unwanted radiation. On the debit side, the termination does not lead to a compact configuration, with the antenna plus, typically, a coaxial connector exhibiting additional depth and bulk. A pin 30 projects from the connector and is typically soldered to the patch. The feed network must lie in a separate layer behind the radiating surface, so the complete antenna cannot be etched on a single substrate.
For modern telecommunications applications, apart from the electrical performance of the antenna other factors need to be taken into account, such as size, weight, cost and ease of construction of the antenna. Depending on the requirements, an antenna can be either a single radiating element or an array of like radiating elements. With the increasing deployment of cellular radio, an increasing number of base stations which communicate with mobile handsets are required. Similarly an increasing number of antennas are required for the deployment of fixed radio access systems, both at the subscribers premises and base stations. Such antennas are required to be both inexpensive and easy to produce. A further requirement is that the antenna structures be of light weight yet of sufficient strength to be placed on the top of support poles, rooftops and similar places and maintain long term performance over environmental extremes.
Typical subscriber antennas for fixed wireless access installations employing patch antennas have microstrip feed cut-ins to find the optimum feed point. Patches having such cut-ins, however, do not necessarily provide good cross polar performance. Also the patch cannot be widened for increased bandwidth, since it needs to be symmetrical, regarding the need for two polarisations. It is therefore very important to minimise parasitic effects of the feed while maintaining simple manufacturability.
The present invention seeks to provide a patch antenna and a feed network therefor. The present invention further seeks to provide a patch antenna of reduced Z-axis dimensions and which can achieve dual polarisation capability and can be matched for a maximum of bandwidth.
In accordance with a first aspect of the invention, there is provided a patch antenna comprising a dielectric substrate having a patch element on a first side in connection with a microstrip feed therefor on a second side of the substrate and a reflector ground plane; wherein the microstrip feed line is connected through the substrate to the patch, whereby the microstrip feed line lies parallel to the patch, with the patch acting as a ground with respect to the microstrip line.
No edge interference is produced due to the coupling of a microstrip line to a surface contact point of the patch. The patches can be rectilinear or ellipsoidal, and can have one or more feeds. Preferably the shielding ground is disposed on the surface of the dielectric which supports the patch element. The patch and ground plane thereby screen the microstrip feed line and distribution network, for any polarisation. This type of feed arrangement can provide an optimum feed point location for any polarisation. In dual polarised mode, there is no compromise in either cross polar performance nor impedance matching.
A matching network can be disposed on the antenna dielectric. Preferably, this network is positioned on an opposite side of the dielectric to and shielded by the ground plane. By the use of microstrip printing techniques a patch antenna can be simply and cost effectively manufactured; fewer process steps are involved in production and microstrip techniques are well developed. The matching network can be formed with discrete components.
In accordance with another aspect of the invention there is provided a method of operating of a patch antenna comprising a patch element, a dielectric substrate, a ground plane and a feed network, the patch antenna element comprising a patch element, a dielectric substrate, a ground plane and a feed network; wherein the patch is supported on a first side of the dielectric substrate and transmits and receives signals via a feed line positioned on the other side of the board opposite the patch element, whereby the signals are transmitted in a microstrip transmission mode.